There are three primary types of interconnect for second level (e.g. substrate to a board), namely: ball grid array (BGA), column grid array (CGA) and land grid array (LGA). These are types of surface-mount packaging commonly used for integrated circuits.
In a BGA, balls of solder are stuck to the bottom of the package. The device is placed on a printed circuit board (PCB) that has copper pads in a pattern matching the solder balls. The assembly is then heated (e.g. in a reflow oven or by an infrared heater), causing the solder balls to melt. Surface tension causes the molten solder to hold the package in alignment with the circuit board, at the correct separation distance, while the solder cools and solidifies. The composition of the solder alloy and the soldering temperature are carefully chosen so that the solder does not completely melt, but stays semi-liquid, allowing each ball of solder to stay separate from its neighbors. Unlike with a pin grid array (PGA) interconnect (a type of interconnect that uses an array of metal pins which are inserted into holes on a PCB), a BGA has a significantly lower chance of accidentally bridging adjacent connects with solder since the solder is factory-applied to the package in exactly the right amount.
A CGA interconnect uses a high-temperature solder column instead of the high-temperature solder ball of the BGA to create a higher standoff for more flexible interconnection and to achieve an increased thermal fatigue life of the package solder joint. See “Ceramic Column Grid Array Package,” Application Note, Actel, September 2003.
In comparison to a PGA, in place of pins a LGA interconnect has pads of bare gold-plated copper that touch pins on the PCB. Interconnection between mating surfaces of a module, or other area array device, and a PCB is provided through a conductive interposer. Connection is achieved by aligning the contact array of the two mating surfaces and the interposer, and mechanically compressing the interposer. See “Land grid array sockets for server applications,” Corbin et al., IBM Journal of Research and Development, Vol. 46, No. 6, November 2002, pp. 763-778.
The use of each of these three technologies is generally dictated by the application. In many cases, larger substrates require migration to an LGA for reliability reasons. The reliability is often dictated by a coefficient of thermal expansion (CTE) mismatch between ceramic substrates and boards which can result in fatigue of the interconnect. For cost reasons, a BGA interconnect may be desirable. However, with the reliability concerns and the increased weight and complexity of thermal solutions, a LGA has become more and more desirable. In addition, a LGA interconnect allows for in-the-field replacement, in the event of a problem with the system.